Modern electronic devices commonly include printed circuit boards reinforced with glass fibers. Many modern electronic devices, such as mobile or stationary wireless telephones, computers, personal data assistants (“PDA's”), and the like, have electronic systems that operate at high or ultra-high frequencies. When glass is exposed to such a high or ultra-high frequency electromagnetic field, the glass absorbs at least some energy and converts the absorbed energy to heat. The energy that is absorbed by the glass in the form of heat is called dielectric loss energy. This dielectric loss energy is in proportion to the “dielectric constant” and the “dielectric loss tangent” of the glass composition, as indicated by the following expression:W=k·f·v2·ε·(tan δ)where “W” is the dielectric loss energy in the glass, “k” is a constant, “f” is the frequency, “v2” is the potential gradient, “ε” is the dielectric constant, and “tan δ” is the dielectric loss tangent. As the above expression indicates, the dielectric loss energy “W” increases with an increase in the dielectric constant and the dielectric loss tangent of the glass, and/or with an increase in frequency.
Two types of glass fibers commonly used to reinforce printed circuit boards are E-glass and D-glass. E-glass, however, has a relatively high dielectric constant ranging from about 6.6 to about 8.1, and a relatively high dielectric loss tangent ranging from about 12×10−4 to about 26×10−4 at a frequency of about 1 MHz at room temperature. Accordingly, because E-glass can yield relatively high dielectric losses, E-glass is a poor reinforcement material for printed circuit boards having higher densities of electronic components and higher processing speeds. D-glass, on the other hand, has a relatively low dielectric constant of about 4.3, and a relatively low dielectric loss tangent of about 10×10−4 at a frequency of about 1 MHz at room temperature. D-glass, however, has relatively high melting temperatures, relatively poor workability, relatively poor mechanical performance, and relatively poor water resistance. In addition, D-glass may inadequately adhere to epoxy resins, and commonly includes imperfections in the form of striae and bubbles. Accordingly, neither E-glass nor D-glass are ideally suited for use as reinforcement fibers in high speed printed circuit boards, and neither is well-suited for circuit boards that operate at high or ultra-high frequencies from about 1 MHz to about 18 GHz.
Others have attempted to develop alternative glass compositions with low dielectric properties that are better suited for use in printed circuit boards for high speed and ultra-high speed electronic devices. For example, Nitto Boseki Co., Ltd. Corporation of Japan produces and markets such an alternative glass composition under the mark NITTOBO NE-GLASS®. Such alternative dielectric glass compositions typically include by percent weight about 45-65% SiO2, about 13-30% B2O3, and about 8-20% Al2O3 as principal constituents. In addition, such glass compositions typically include a combination of at least some MgO and/or at least some CaO as a flux to decrease viscosity and ease melting. When used in combination, the total amount of MgO and/or CaO typically is included in amounts of at least 4 percent by weight. For example, one such composition containing at least about 4 percent by weight of MgO and/or CaO is described in published U.S. Patent Application No. US2003/0054936A1, assigned to Nitto Boseki Co., Ltd. The presence of MgO, however, can increase batching costs, cause undesirable phase separation, decrease water resistance, increase the dielectric constant and the dielectric loss tangent of the glass composition to unacceptable levels, and, if introduced as the raw material calcium magnesium carbonate (“dolomite”), can cause decrepitation. Accordingly, a glass fiber that includes at least about 4 percent CaO by weight as a fluxing agent, and substantially no MgO, would be desirable.
The newer alternative dielectric glass compositions described above also commonly include Li2O, Na2O, and/or K2O as fluxing agents in a total amounts less than about 0.5 percent by weight. The presence of one or more of these constituents, however, can cause the dielectric loss tangent of the glass composition to increase, and may decrease the glass composition's water resistance.
TiO2 is also commonly included in alternative dielectric glass fibers to decrease viscosity, and decrease the dielectric loss tangent. The presence of TiO2, however, can result in phase separation, decrease the chemical durability of the resultant glass fiber, and impart n undesirable yellow tint to the glass.
Accordingly, a dielectric glass fiber that includes substantially no MgO, substantially no Li2O, Na2O, and/or K2O, and substantially no TiO2, would be desirable.